You can suppress those warnings if you want as follows:

There is a way to suppress them by finding the .cbb file for that backup job.
To find the .cbb files do the following: Tools -> Diagnostics -> Open in Folder and go to the folder that is one layer up.
The cbb files all have a rubbish name, so what I do is Edit and save the plan again and then sort the folder by datetime. The first .cbb is the one you need.

Change the <UseIgnoreErrorsOnPathNotFound>false</UseIgnoreErrorsOnPathNotFound> line to <UseIgnoreErrorsOnPathNotFound>true</UseIgnoreErrorsOnPathNotFound> with the help of any text editor and save the file.